The present invention is in the field of photosensitive films for optical information recording, and particularly relates to films which can exhibit both high levels of induced birefringence and relatively high transmittance at near infrared light wavelengths.
The fact that optical bleaching with polarized light can induce dichroism and birefringence in silver-containing silver halide photographic emulsions has long been known, being reported by Cameron and Taylor in "Photophysical Changes in Silver-Silver Chloride Systems", J.O.S.A. Vol. 24, Pages 316-330 (1934). More recently, analogous affects in silver halide-containing glasses have been observed, as reported by R. J. Araujo et al. in U.S. Pats. Nos. 4,125,404 and 4,125,405.
The particles responsible for the effects observed in these systems are referred to as additively colored silver halide crystals. These are silver halide crystals containing or associated with metallic silver, the silver metal acting to absorb visible light and being permanently bleachable by light of appropriate wavelength and intensity.
Other workers have reported optically induced dichroic effects in additively colored silver halide films, including V. P. Cherkashin in Soviet Physics State, Vol. 13, No. 1, pp. 264-265 (1971), and L. A. Ageev et al. in Opt. Spektrosk, Vol. 40, pp. 1024-1029 (June 1976). In our French Pat. No. 2,370,303 we disclose multilayer photosensitive films consisting of alternating layers of a dielectric acceptor such as a silver halide and a metal such as silver which are useful for optical information storage. These are light-absorbing films which can be optically bleached and which retain information relating to the color, intensity and polarization of the bleaching light.
Even though films of the type described in the foregoing studies comprise light-alterable silver halide crystals, their characteristics are substantially different from the characteristics of conventional silver halide photographic films. Additively colored films are light-absorbing as made, and are visibly bleached by the action of visible light. In addition, no chemical treatments are required for the development or preservation of the various bleaching effects which have been observed.
Conventional photographic materials could perhaps be used for optical information storage applications such as laser-implemented recording processes, but such materials undesirably require chemical development of the recording to amplify and fix the recording image. This characteristic renders them unsuitable for many optical recording applications. The requirements for a medium to be used for high density optical information storage have previously been defined, being described, for example, by R. A. Bartolini et al. in IEEE Spectrum, pp. 20-28 (August 1968). The obvious requirements are high writing sensitivity, high spot resolution and acceptable readout efficiency. Additional characteristics which are clearly desirable are a capability for reuse and the absence of any requirement for a post-exposure image intensification or fixing step.
Optical recording media comprising thin films of an evaporable metal, such as described by Bartolini et al., supra, satisfy most of these requirements but are not reusable. Another category of films which has been considered for optical recording includes the magneto-optic films such as MnBi, discussed by R. W. Cohen et al. in "Materials for Magneto-Optic Memories", RCA Review, Vol. 33, pp. 54-70 (March 1972). However, further improvements in the signal-to-noise ratios of these materials would be desirable.